Acidic CO2-to-HCOOH electrolysis with industrial-level current on phase engineered tin sulfide

Acidic CO2-to-HCOOH electrolysis represents a sustainable route for value-added CO2 transformations. However, competing hydrogen evolution reaction (HER) in acid remains a great challenge for selective CO2-to-HCOOH production, especially in industrial-level current densities. Main group metal sulfides derived S-doped metals have demonstrated enhanced CO2-to-HCOOH selectivity in alkaline and neutral media by suppressing HER and tuning CO2 reduction intermediates. Yet stabilizing these derived sulfur dopants on metal surfaces at large reductive potentials for industrial-level HCOOH production is still challenging in acidic medium. Herein, we report a phase-engineered tin sulfide pre-catalyst (π-SnS) with uniform rhombic dodecahedron structure that can derive metallic Sn catalyst with stabilized sulfur dopants for selective acidic CO2-to-HCOOH electrolysis at industrial-level current densities. In situ characterizations and theoretical calculations reveal the π-SnS has stronger intrinsic Sn-S binding strength than the conventional phase, facilitating the stabilization of residual sulfur species in the Sn subsurface. These dopants effectively modulate the CO2RR intermediates coverage in acidic medium by enhancing *OCHO intermediate adsorption and weakening *H binding. As a result, the derived catalyst (Sn(S)-H) demonstrates significantly high Faradaic efficiency (92.15 %) and carbon efficiency (36.43 %) to HCOOH at industrial current densities (up to −1 A cm−2) in acidic medium.

The manuscript by Shen et al. demonstrates a phase engineering strategy to steer the construction of Sn active sites toward acidic CO2-to-HCOOH process. In situ characterizations and theoretical calculations evidence the stronger Sn-S binding strength on π-SnS phase than that on the conventional α-SnS phase, facilitating the stabilization of residual S species. The S dopants finally contribute to the efficient and highly selective CO2 conversion to HCOOH over derived Sn(S)-H catalyst. The presented results are interesting given that the importance of acidic CO2RR subfield. Mechanically, the identification or manipulation of active sites is a central topic in catalysis science, and the CO2RR catalyst stability deserves an in-depth concern especially in acidic medium. However, there are some inadequacies required to be first clarified; therefore I would like to recommend a major revision before acceptance to address the concerns listed below: 1. From my perspective, the sulfate ions in acidic electrolyte could disturb the precise analysis of S element in Sn(S)-H catalyst after activation step (NEXAFS and XPS in Fig. 2e, f), although it is claimed by authors that sulfate ions and Sn-S can be distinguished. In this case, an acidic electrolyte without S element could be better for reaching a more convincing conclusion. Besides, according to XRD in Fig. 2a, SnS phase almost disappears in metallic Sn(S)-H catalyst, while for Raman spectra in Fig. 2d, some SnS phase still exists even after 10 h at -1.5 V (vs RHE). How to explain this seeming contradiction? 2. In Fig. 3d, the stability test lasts only for 260 min, which hardly meets the industrial demands. Therefore, the authors should attempt to prolong the test, and also provide the TEM image of Sn(S)-H after durability test to inspect its structural integrality. In addition, is S element stable in Sn(S)-H catalyst after activation step and during electrochemical CO2RR process? Experimentally, what is the role of acidic environment on influencing the reconstruction of π-SnS or Sn(S)-H catalyst during activation or CO2RR working process?
3. The authors pay attention on inhibiting the sulfur dissolution, which is also considered by another latest work (ACS Catal. 2022, 12, 13533-13541). However, for acidic CO2RR, I would stress that metal dissolution should be more important in terms of catalyst stability in acidic medium. Given that the redox potential of Sn element, I recommend performing ICP test of the electrolyte to examine the possible Sn dissolution after acidic CO2RR operation. On the other hand, does the remained S element affect the stability of Sn(S)-H or Sn(S)-L catalyst? This might be answered by comparing the stability performance of Sn(S)-H or Sn(S)-L. Furthermore, the DFT calculations can give some proofs by evaluating the parameters such as vacancy formation energy difference of Sn atom while after introducing S element.
4. I suggest that authors should discreetly check the Raman data in Supplementary Fig. 19, especially for the analysis on symmetric/asymmetric stretching vibration (νsO-C-O) of *OCHO intermediate at 1350 and 1580 cm-1, claimed by them. These two bands are more likely attributed to the D and G band of graphene or carbon materials (Nature Nanotechnol., 2013, 8(4): 235-246), as a carbon-based GDE is used during in-situ electrochemical measurements. 5. I understand that the authors attribute the catalytic performance difference to the S element for Sn(S)-H or Sn(S)-L. Despite this, they should try to exclude other factors, such as defects, or coordination environment of active sites. In atomic-resolution HAADF-STEM ( Fig. 2b and Supplementary Fig. 7), the exposed facet with corresponding lattice parameters should be labeled to make a clear comparison. By the way, the whole manuscript should be carefully checked to correct those possible expression errors. Finally, I suggest incorporating more related literatures to enrich the background or discussion on structural reconstruction or catalyst stability topic, such as Adv. Funct. Mater. 2022, 32, 2111193;Nano Res., 2022, 15(4): 3283-3289;Adv. Energy Mater. 2022, 12, 2200970, etc. Reviewer #2 (Remarks to the Author): The authors report a phase engineering strategy of π-SnS that can stabilize rich S dopants on Sn subsurface in acidic medium for efficient CO2-to-HCOOH production.
The π-SnS derived S-doped Sn catalyst achieves a high FE (over 70 %) of HCOOH production. The topic is interesting and the results are reliable. It might be accepted after the following issues are addressed.
1. It is stated that "As shown in Fig. 4a, the distinct *OCHO signal around 1367 cm−1 on Sn(S)-H confirms the efficient HCOOH generation." Actually, the *OCHO hydrogenation is possible to generate other intermediate.
2. I recommend to check the adsorption energy of *COOH on the surface, and the reaction path *CO2→*COOH→…..should be shown in the Free energy diagram of CO2 reducfion.

Reviewer's Remarks to Authors
The manuscript by Shen et al. demonstrates a phase engineering strategy to steer the construction of Sn active sites toward acidic CO2-to-HCOOH process. In situ characterizations and theoretical calculations evidence the stronger Sn-S binding strength on π-SnS phase than that on the conventional α-SnS phase, facilitating the stabilization of residual S species. The S dopants finally contribute to the efficient and highly selective CO2 conversion to HCOOH over derived Sn(S)-H catalyst. The presented results are interesting given that the importance of acidic CO2RR subfield.
Mechanically, the identification or manipulation of active sites is a central topic in catalysis science, and the CO2RR catalyst stability deserves an in-depth concern especially in acidic medium. However, there are some inadequacies required to be first clarified; therefore I would like to recommend a major revision before acceptance to address the concerns listed below.

Response
We thank Reviewer #1 for his/her valuable comments and positive recommendation. Raman spectra in Fig. 2d, some SnS phase still exists even after 10 h at - 1.5 V (vs RHE). How to explain this seeming contradiction?

Response
We agree with Reviewer #1 that the S species in potassium sulfate and sulfuric acid may affect the spectroscopic data. Therefore, we conducted a control experiment using potassium perchlorate and perchloric acid as alternatives. We didn't use phosphoric acid and nitric acid because these two acids reveal either incomplete ionization or NO3 − reduction reaction on the cathode. However, due to the low solubility of potassium perchlorate, we used saturated potassium perchlorate (around 0.12 M) mixed with perchloric acid (pH = 3) solution and 1 M sodium perchlorate mixed with perchloric acid (pH = 3) solution, respectively. Figure R1, π-SnS derived catalysts (Sn(S)-H) illustrate higher residual S amount than α-SnS derived catalysts (Sn(S)-L) in both saturated KClO4 + HClO4 electrolyte and 1 M NaClO4 + HClO4 electrolyte, confirming the stronger Sn-S binding strength on π-SnS than α-SnS.
In response to address this comment, we have in our: 1) R-SI, added Figure R1 as Supplementary Fig. 9 with the following clarifying note; 2) R-MS, p. 10, para. 1, included the additional text as follows; 'The similar evidence of higher residual S amount in Sn(S)-H could also be found in other sulfurfree acidic electrolytes (Supplementary Fig. 9).' The XRD mainly detects the crystal structure of the material. However, the subsurface residual S-Sn bonds in Sn(S)-H after activation can't be detected by XRD because the reconstruction induced lattice collapse. Therefore, we use in situ Raman spectroscopy to detect the vibration of residual S-Sn bond (Nat. Protoc. 2023, https://doi.org/10.1038/s41596-022-00782-8). Compared with XRD, in situ Raman spectroscopy has a much lower detection depth but is more surface sensitive even though there is no long-range ordered SnS lattice. Therefore, after activation under −0.1 A cm −2 for 900 s, XRD reveals a metallic Sn structure in Sn(S)-H, while the residual S species in the Sn subsurface (Sn-S) are detected by in situ Raman spectra.

Comment 1-2
In Fig. 3d, the stability test lasts only for 260 min, which hardly meets the industrial demands.
Therefore, the authors should attempt to prolong the test, and also provide the TEM image of

Sn(S)-H after durability test to inspect its structural integrality. In addition, is S element stable in Sn(S)-H catalyst after activation step and during electrochemical CO2RR process?
Experimentally, what is the role of acidic environment on influencing the reconstruction of π-SnS or Sn(S)-H catalyst during activation or CO2RR working process?

Response
We found that the stability of flow cells is affected by the potassium sulfate deposition on the gasdiffusion layer (Nat. Catal., 2022, 5, 564-570.). Therefore, we conducted the test by periodically removing the potassium sulfate deposition and refreshed the electrolyte for both Sn(S)-H and Sn(S)-L. In addition, the volume of the cathode and anode electrolytes are changed to 2.2 L and 1 L, respectively, to ensure a stable electrocatalytic environment. As seen in Figure R3, Sn(S)-H maintained a high FE of HCOOH (over 85%) after 13.5 h under a current density of −400 mA cm −2 . Further HAADF-STEM images of Sn(S)-H confirm the stable Sn structure after stability measurement ( Figure R4).
In addition, both EDS mapping and XPS spectra of Sn(S)-H after stability measurement confirm the stable S-Sn bonding on Sn(S)-H, shown in Figure R4b and Figure R5a. For comparison, the Sn(S)-L shows a trace amount of S-Sn bond after stability measurement, which is much lower than that in Sn(S)-H, Figure R5b.
In response to address this comment, we have in our: 1) R-MS, put Figure R3 as   To understand the role of acidic environment in the reconstruction of π-SnS, we used neutral (0.5 M K2SO4) and alkaline electrolyte (1M KOH) to detect the catalysts evolution. As demonstrated in Figure R6, the XPS spectrum of the sample derived in neutral electrolyte reveals a higher residual S (Sn-S) amount than that in acidic and alkaline environments. The lower residual S species in Sn(S)-H derived from acidic or alkaline environment could relate to the fast hydrogen evolution reaction (HER) dynamics, which accelerates the S dissolution during the reconstitution.
However, acidic media is ideal for HCOOH production and was used as the electrolyte in this work.

Comment 1-3
The authors pay attention on inhibiting the sulfur dissolution, which is also considered by another latest work (ACS Catal. 2022, 12, 13533-13541

Response
Thanks for the valuable suggestion. We took the ICP test for the electrolyte at different reaction time for Sn(S)-H and Sn(S)-L ( Figure R7). In contrast to Sn(S)-L, Sn(S)-H demonstrated a lower dissolving rate, proving that the strong S-Sn bond can prevent the dissolution of Sn in Sn(S)-H. Figure R8 shows the stability performance of Sn(S)-H and Sn(S)-L, the FE of HCOOH in Sn(S)-L decreased to 40% after 6 h, while Sn(S)-H maintained a high FE of HCOOH (over 85%) after 13.5 h, which could be attribute to the remained S element. Therefore, the lower Sn dissolving rate in Sn(S)-H confirms the positive effect of S for acid resistance.

Comment 1-4
I suggest that authors should discreetly check the Raman data in Supplementary Fig. 19 Figure R9. In addition, the *OCHO intermate is widely believed as the main intermate of formate/formic acid production for Sn and Sn-based materials (Joule, 2017, 1, 794-804;Nat. Comm., 2022, 13, 2486Angew. Chem. Int. Ed., 2021, 60, 26233-26237), which is further confirmed in our in situ ATR-FTIR spectra (Fig. 4a, b and Supplementary Fig. 25). Therefore, we believe that deleting these two labels does not affect the accuracy of the conclusion.
In response to address directly this comment, we have in our: 1) In R-SI, revised Supplementary Fig. 24 to delete the marks of the broad peaks around 1350 and 1590 cm -1 .

Response
We conducted additional HAADF-STEM measurements to explore the influence of defects, vacancies and coordination environment on the electrocatalysts (Adv. Funct. Mater., 2022, 32, 2111193;Angew. Chem. Int. Ed., 2021, 60, 18178-18184;Nat. Comm., 2021, 12, 660). We found that no obvious defects were observed in both Sn(S)-H and Sn(S)-L ( Figure R10). Thus, we believe the main contribution of the catalytic performance difference for Sn(S)-H and Sn(S)-L is the different residual S amount.
In response to address this, we have, in our: 1) R-SI, added Figure R10 as Supplementary Fig. 10.
2) R-MS, p.10, para. 1, included the following clarifying text; To make a clear comparison of the exposed facet for Sn(S)-H and Sn(S)-L, we use the magnified HAADF-STEM images of Sn(S)-H and Sn(S)-L as shown in Figure R11 and Figure   R12. The exposed facets of Sn(S)-H and Sn(S)-L in Figure R11 and Figure R12 were determined as Sn (200), which is consistent with the XRD results. However, according to HAADF-STEM images, both Sn(S)-H and Sn(S)-L illustrated polycrystalline structures after reconstruction. We find Sn (220) and Sn (101)    We agree with Reviewer #1 to enrich the content of this article in background and discussion on structural reconstruction or catalyst stability, as structural reconstruction and catalyst stability are challenging areas with constant attention in current research of CO2 reduction.

Reviewer's Remarks to Authors
The authors report a phase engineering strategy of π-SnS that can stabilize rich S dopants on Sn subsurface in acidic medium for efficient CO2-to-HCOOH production.
The π-SnS derived S-doped Sn catalyst achieves a high FE (over 70 %) of HCOOH production.
The topic is interesting and the results are reliable. It might be accepted after the following issues are addressed.

Response
We thank Reviewer #2 for his/her valuable comments and positive recommendation for publication.

Comment 2-1
It is stated that "As shown in Fig. 4a

Response
We checked other possible intermediates after *OCHO hydrogenation. For the two *O atoms binding on the surface, further hydrogenation of *OCHO does not generate multi-carbon products as C-C coupling normally needs *C for binding atoms. Therefore, HCOOH is the thermodynamically optimal product, and H2COOH* and H2CO* are two possible intermediates to generate HCHO, CH3OH or CO (Nat. Chem., 2014, 6, 320;Angew. Chem. Int. Ed., 2018, 57, 15045-15050;Nat. Nanotechnol., 2021, 16, 1386-1393. According to the product detection and in situ experiments, no obvious H2COOH* and H2CO* intermediates are detected. Thus, we confirmed that HCOOH is the main product for *OCHO hydrogenation. To improve the rigor of the article, we have: In R-MS, p. 14, para. 3, revised the text as follows; 'For two CO2-to-HCOOH intermediates, *OCHO is widely considered more efficient than *COOH for HCOOH production. As shown in Fig. 4a

Response
The adsorption energy of *COOH on the surface has been checked. The comparison in adsorption free energy of *COOH and *OCHO is shown in Figure 4d. In addition, we agree that a direct comparison of the reaction pathway via *OCHO and *COOH shows the reaction selectivity mechanism more intuitively. As shown in Figure R14, these two reaction pathways are plotted together. It can be seen that the *OCHO pathway is energetically more favorable than *COOH.
This well confirms that the reaction selectively produces HCOOH rather than CO.
In response to address this, we have, in our: 1) R-SI, added Figure R14 as Supplementary Fig. 27.
2) R-MS, p.17, para. 2, included the following clarifying text: 'The pathway via *OCHO is energetically more favorable than *COOH (Supplementary Fig. 27), which confirms that the reaction selectively produces HCOOH rather than CO.' (through *OCHO) is significantly more favorable than the CO pathway (through *COOH).

Comment 2-3
According to the *OCHO with two O atoms binding with the surface, rather than the proton *H, the authors conclude that S-doped Sn promotes CO2RR to produce HCOOH but suppresses HER.